Abstract
AbstractNitric oxide (NO) produced in the polar middle and upper atmosphere by energetic particle precipitation depletes ozone in the mesosphere and, following vertical transport in the winter polar vortex, in the stratosphere. Medium‐energy electron (MEE) ionization by 30–1,000 keV electrons during geomagnetic storms may have a significant role in mesospheric NO production. However, questions remain about the relative importance of direct NO production by MEE at altitudes ~60–90 km versus indirect NO originating from auroral ionization above 90 km. We investigate potential drivers of NO variability in the southern‐hemisphere mesosphere and lower thermosphere during 2013–2014. Contrasting geomagnetic activity occurred during the two austral winters, with more numerous moderate storms in the 2013 winter. Ground‐based millimeter‐wave observations of NO from Halley, Antarctica, are compared with measurements by the Solar Occultation For Ice Experiment (SOFIE) spaceborne spectrometer. NO partial columns over the altitude range 65–140 km from the two observational data sets show large day‐to‐day variability and significant disagreement, with Halley values on average 49% higher than the corresponding SOFIE data. SOFIE NO number densities, zonally averaged over geomagnetic latitudes −59° to −65°, are up to 3 × 108/cm3 higher in the winter of 2013 compared to 2014. Comparisons with a new version of the Whole Atmosphere Community Climate Model, which includes detailed D‐region ion chemistry (WACCM‐SIC) and MEE ionization rates, show that the model underestimates NO in the winter lower mesosphere whereas thermospheric abundances are too high. This indicates the need to further improve and verify WACCM‐SIC with respect to MEE ionization, thermospheric NO chemistry, and vertical transport.
Highlights
A smoothly varying baseline of ~73 K has been subtracted from the atmospheric spectrum by fitting a four-term polynomial to the data excluding the region within ±1.5 MHz of the Nitric oxide (NO) peak
NO in the mesosphere and lower thermosphere above Antarctica has been studied during a two-year period, 2013–2014, close to solar maximum
Contrasting levels of geomagnetic storm activity occurred during the two austral winters, with nine moderate geomagnetic storms during May–August in 2013 compared to just one storm at the start and one close to the end of the corresponding period in 2014
Summary
NOx exists mainly as NO in the thermosphere and upper mesosphere and is converted to NO2 below 65 km (Solomon et al, 1982) Enhanced abundances of these chemical species lead to catalytic destruction of ozone (Jackman & McPeters, 2004), perturbing the radiative balance, dynamics, and large-scale circulation patterns of the atmosphere. This mechanism potentially links solar variability associated with space weather to regional surface climate (e.g., Arsenovic et al, 2016; Baumgaertner et al, 2011; Semeniuk et al, 2011; Seppälä et al, 2009, 2013). At geomagnetic latitudes of 70°–75° high fluxes of low-energy (1–30 keV) auroral electrons enter the atmosphere almost continuously and produce abundant nitric oxide (NO) in the lower thermosphere at 100–120 km, even during low geomagnetic activity (Marsh et al, 2004)
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